1. Field of the Invention
The present invention relates to a variable focal length lens system and an image pickup device, and particularly to the technical field of a variable focal length lens system and an image pickup device that are used in a video camera, a digital still camera and the like, whose angle of view in a wide-angle end state exceeds 74 degrees, and whose variable power ratio exceeds 30 times.
2. Description of the Related Art
In the past, a method that converts an amount of light of a subject image formed on an image pickup element surface into electric output by an image pickup element using a photoelectric conversion element such as a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor) or the like, and which records the electric output is known as recording means in a camera.
With the recent progress of microfabrication techniques, central processing units (CPU) have been increased in speed and storage media have been increased in degree of integration, so that high-volume image data that has heretofore been unable to be handled can be processed at higher speed. In addition, light receiving elements have been increased in degree of integration and reduced in size. The increased degree of integration has enabled recording of high spatial frequencies, and the reduced size has make it possible to miniaturize the camera as a whole.
However, the increased degree of integration and the reduced size as described above decreases the light receiving area of each individual photoelectric conversion element (light receiving element), and effect of noise is increased as electric output is decreased. Accordingly, in order to reduce such noise effect, an amount of light arriving on the light receiving element may be increased by a higher aperture ratio of an optical system. In addition, a minute lens element referred to as a microlens array may be disposed immediately in front of the light receiving element.
The microlens array leads a luminous flux going between elements adjacent to each other onto the light receiving elements, but limits the exit pupil position (distance from an image surface to the exit pupil position) of a lens system. This is because when the exit pupil position of the lens system approaches a light receiving element, an angle formed between a principal ray reaching the light receiving element and an optical axis is increased, thus an off-axis luminous flux going toward a peripheral part of a screen forms a great angle with the optical axis, and as a result, a necessary amount of light does not reach the light receiving element, thus inviting a lack of an amount of light.
User needs have recently been diversified as digital cameras spread. There is particularly a desire for a camera reduced in size and including a zoom lens having a high variable power ratio (variable focal length lens system).
A four-group configuration of a positive group, a negative group, a positive group, and a positive group has conventionally been used as a type representing a configuration of a variable focal length lens system having a high variable power ratio in the past.
The variable focal length lens system of the four-group configuration of a positive group, a negative group, a positive group, and a positive group is formed by arranging a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power in order from an object side to an image side.
A variable focal length lens system described in Japanese Patent Laid-Open No. 2006-189598 (referred to as Patent Document 1 hereinafter), for example, is known as a variable focal length lens system of the four-group configuration of a positive group, a negative group, a positive group, and a positive group as described above.
It is generally known that as the number of movable lens groups is increased, a degree of freedom of selection of the locus of each lens group at a time of zooming from a wide-angle end state to a telephoto end state is increased, and it is therefore possible to achieve a higher variable power ratio and achieve higher performance as well.
Variable focal length lens systems described in Japanese Patent Laid-Open No. 2007-79194 and Japanese Patent Laid-Open No. 2007-292994 (hereinafter referred to as Patent Document 2 and Patent Document 3, respectively), for example, are known as variable focal length lens systems that achieve such a higher variable power ratio and such higher performance.
The variable focal length lens systems described in Patent Document 2 and Patent Document 3 have a configuration in which a fifth lens group fixed in the direction of an optical axis is disposed on the image side of a variable focal length lens system of a four-group configuration of a positive group, a negative group, a positive group, and a positive group.
In addition, a variable focal length lens system of a five-group configuration of a positive group, a negative group, a positive group, a negative group, and a positive group described in Japanese Patent Laid-Open No. 2007-264174 (referred to as Patent Document 4 hereinafter), for example, is known as a variable focal length lens system of another five-group configuration. The variable focal length lens system of the five-group configuration of a positive group, a negative group, a positive group, a negative group, and a positive group is formed by arranging a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having negative refractive power, and a fifth lens group having positive refractive power in order from an object side to an image side.
In the variable focal length lens system described in Patent Document 4, when a lens position state changes from a wide-angle end state to a telephoto end state, the first lens group and the third lens group move to the object side, the second lens group moves to the object side after once moving to the image side, the fourth lens group is fixed in the direction of an optical axis, and the fifth lens group moves in the direction of the optical axis.